That messy state may include a new, partially-implemented module file. By default `git stash` will leave it alone, because it's untracked. But `git stash` will also remove its corresponding `mod` declaration, and any uses of it, all at once.
Now say you want to build and run. If that WIP module file gets picked up automatically, it will probably break the build! It may simply be incomplete, or it may depend on some of those stashed changes, or whatever else. Even if it happens to typecheck, now you're assuming that it's all dead code- but it may have some public API (including trait impls!), and still be included in the build!
In fact, we already have a taste of this kind of problem, ever since Cargo started running build.rs scripts automatically, even without a `package.build` key in `Cargo.toml`. If you add a build script with some build-dependencies, run `git stash`, and then build, the build script will fail to compile!
Perhaps you could argue that this is a deficiency in `git stash` that Rust shouldn't have to be concerned with. But if `git stash` removed untracked files as well, that would simply break other use cases instead! For example, I often have a few extra files laying around, which I don't ever intend to commit or .gitignore- sample inputs or outputs, short-term todo lists, or other scratch-pad-like data. Ideally `git stash` would leave those alone- but it's hard for git to tell those apart from the problematic cases above.